1. Field of the Invention
The present invention relates to a method and apparatus for transmitting data based on Orthogonal Frequency Division Multiplexing (OFDM), for reducing power consumption.
2. Discussion of Related Art
Next-generation very high-speed wireless communication systems are under development for the purpose of realizing high-speed data transmission of more than 1 Gb/s in a local area. Currently influential local area very high-speed wireless communication network technology includes a next-generation Wireless Local Area Network (WLAN), which is under research as a standard of IEEE 802.11n, and a high-speed Wireless Personal Area Network (WPAN), which is under development as a standard of IEEE 802.15.3a. The next-generation WLAN guarantees an actual throughput of more than 100 Mb/s in a Media Access Control (MAC) layer.
The next-generation WLAN basically uses Orthogonal Frequency Division Multiplexing (OFDM) in a physical layer so as to maintain backward compatibility with a conventional WLAN based on an IEEE 802.11a/g standard. The OFDM has good spectrum efficiency due to mutual orthogonality and superposition of spectrums of a sub-channel.
The high-speed WPAN uses a signal transmission physical layer, such as ultra-wideband OFDM using a base-bandwidth of 528 MHz and Direct Sequence-Ultra WideBand (DS-UWB), which are influential international standard technologies. In the Direct Sequence-Ultra WideBand (DS-UWB), transmission data is multiplied by a spread code, and a bandwidth signal is converted to have an ultra wide bandwidth of more than 3 GHz in a Direct-Sequence Spread Spectrum (DSSS).
Detailed descriptions of the above methods are made with reference to data of a working group for IEEE 802.15.3a standardization.
FIG. 1 is a block diagram illustrating a conventional transceiving system based on OFDM.
Referring to FIG. 1, an OFDM transmitter 100 includes an OFDM modulator 100a for forming and OFDM-modulating a packet from a payload data signal input through an upper application layer and a MAC layer; and an analog/RF unit 100b for converting a digital signal into an analog signal through digital-to-analog modulation, and transmitting an electronic wave signal through an RF terminal and an antenna.
In detail, the OFDM modulator 100a includes a scrambler 102 for transforming input data; encoders 104 for encoding the input data; interleavers 106 for reducing error between neighboring bits; a mapper 108 for transforming bit stream data into complex number data on an I-Q constellation; a pilot insert unit 110 for adding a pilot signal necessary for channel estimation of an OFDM receiving terminal; an Inverse Fast Fourier Transform (IFFT) unit 112 for transforming a frequency domain signal into a time domain signal by fourier transform; and a guard interval and cyclic prefix insert unit 114 for adding an appropriate Guard Interval (GI) and Cyclic Prefix (CP) signal for corresponding to multi-path fading.
The analog/RF unit 100b includes a wave shaping unit 116 for mixing a preamble signal with an OFDM symbol signal and selectively filtering the mixed signal to reduce a spurious signal; a Digital-to-Analog Converter (DAC) 118 for converting the digital signal into the analog signal; a BaseBand Analog (BBA) unit 120 for filtering and amplifying the analog signal; an RF up-conversion mixer array 122 for converting the amplified signal into an RF signal; an amplifier 124; an antenna 126; and a frequency synthesizer 128 for generating a frequency signal.
An OFDM receiver 130 includes an RF/digital unit 130a for amplifying an electronic wave signal received through an antenna 132 and converting the amplified signal into a digital signal; and an OFDM demodulator 130b for OFDM demodulating data. In detail, the RF/digital unit 130a includes an amplifier 134 for amplifying the signal received through the antenna 132; an RF down-conversion mixer array 136 for converting the amplified signal into a base-band signal; a low pass filter 138 for filtering the base-band analog signal; an Analog-to-Digital Converter (ADC) 140 for converting the analog signal into the digital signal; an Automatic Gain Control (AGC) unit 142 for controlling a gain of a receiving terminal to be suitable for digital signal processing such as frame synchronization, symbol synchronization, and the like; and a synchronizer 144 for performing the signal processing.
The OFDM demodulator 130b includes a guard interval and cyclic prefix eliminator 146 for eliminating a Guard Interval (GI) and Cyclic Prefix (CP) signal; a Fast Fourier Transform (FFT) unit 148 for transforming a time domain signal into a frequency domain signal; an equalizer 150 for amplifying or inserting a signal that is distorted through amplification or transmission into a transmission line for correction and collecting and normalizing a characteristic of the signal by fourier transform; a carrier phase and time tracer 152 connected to the FFT unit 148 and the equalizer 150 and tracing a phase and a time of a carrier; a demapper 154 for again transforming complex number data into bit stream data; a deinterleaver 156; a decoder 158 for decoding data; and a descrambler 160 for descrambling data.
However, in the case where a very high-speed wireless communication base-band modem chip having a data transfer rate of 1 Gb/s is realized using a conventional OFDM transceiver, there is a drawback in that a clock speed for driving a digital chip, a sampling clock speed of the digital-to-analog converter 118 and the analog-to-digital converter 140, and a bandwidth of the RF amplifiers 124 and 134 are increased, thereby increasing power consumption.
Accordingly, in the case where transmission is performed at a low data transfer rate of 1 Mb/s, using the very high-speed wireless communication base-band modem chip having a data transfer rate of 1 Gb/s, without any consideration of power consumption, an excessive amount of power is consumed. For example, when a low-rate audio signal is transmitted using a chip manufactured using wideband OFDM suitable for a data transfer rate of 1 Gb/s, much more power is consumed compared to a chip optimized for low throughput, such as a Bluetooth chip. In other words, management of power consumption based on the data transfer rate, which has not yet been seriously considered in a conventional wireless communication system having a data transfer rate of less than 100 Mb/s, becomes more important due to an increased data transfer rate of more than 1 Gb/s and increased power consumption.
At present, technology for reducing power consumption includes a method of using a standby mode and/or a sleep mode for deactivating a chip and placing it in an idle state where data is not transmitted so that only minimal power is consumed, and a clock gating and DVFS (Dynamic Voltage Frequency Scaling) method for lowering a clock speed of a processor chip (U.S. Pat. No. 6,609,209).
However, the former method has a drawback in that it takes considerable time to convert an entire transmission system from an active state to a sleep or standby state, and vice versa.
Further, the former method has another drawback in that like a voice/audio signal, a low data transfer rate is required, but it is difficult to apply to Synchronous Connection Oriented (SCO) or streaming systems which are maintained in an active state for the duration of a call or audio reproduction.
The clock gating method is suitable for a computer or a digital processor, but has a drawback in that it is difficult to apply to a wireless transmission system including a plurality of analog/RF circuits that operate independent of a clock signal.
In a communication system, unlike a computer, clock and timing information is very important and is designed in a chip development stage by selecting clocks optimized to a standard of communication. Therefore, in order to vary a clock signal, it is not only necessary to change the entire structure of the communication system, but the standard must be changed as well. Accordingly, broadband low power consumption technology is required to efficiently transmit low-speed streaming data in a broadband transmission system for performing very high-speed wireless communication with a data transfer rate of 1 Gb/s.